Method and apparatus for video coding

ABSTRACT

Aspects of the disclosure include methods, apparatuses, and non-transitory computer-readable storage mediums for video encoding/decoding. An apparatus includes processing circuitry that decodes prediction information of a current block in a current picture that is a part of a coded video sequence. The prediction information indicates a cross component filtering (CCF) process for the current block. The processing circuitry generates filtered reconstruction samples of the current block by applying the CCF process on at least one of predicted samples, residual values, or reconstruction samples of the current block. The filtered reconstruction samples of the current block are used for reconstruction of a subsequent block. The processing circuitry reconstructs the current block and the subsequent block based on the filtered reconstruction samples of the current block.

INCORPORATION BY REFERENCE

This present application claims the benefit of priority to U.S.Provisional Application No. 63/079,322, “CROSS-COMPONENT FILTERING ONBLOCK-LEVEL RECONSTRUCTION,” filed on Sep. 16, 2020, which isincorporated by reference herein in its entirety.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate) of, for example, 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce the aforementioned bandwidth or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless and lossy compression, as well as a combination thereof can beemployed. Lossless compression refers to techniques where an exact copyof the original signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signals is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision distribution applications. The compression ratio achievablecan reflect that: higher allowable/tolerable distortion can yield highercompression ratios.

A video encoder and decoder can utilize techniques from several broadcategories, including, for example, motion compensation, transform,quantization, and entropy coding.

Video codec technologies can include techniques known as intra coding.In intra coding, sample values are represented without reference tosamples or other data from previously reconstructed reference pictures.In some video codecs, the picture is spatially subdivided into blocks ofsamples. When all blocks of samples are coded in intra mode, thatpicture can be an intra picture. Intra pictures and their derivationssuch as independent decoder refresh pictures, can be used to reset thedecoder state and can, therefore, be used as the first picture in acoded video bitstream and a video session, or as a still image. Thesamples of an intra block can be exposed to a transform, and thetransform coefficients can be quantized before entropy coding. Intraprediction can be a technique that minimizes sample values in thepre-transform domain. In some cases, the smaller the DC value after atransform is, and the smaller the AC coefficients are, the fewer thebits that are required at a given quantization step size to representthe block after entropy coding.

Traditional intra coding such as known from, for example MPEG-2generation coding technologies, does not use intra prediction. However,some newer video compression technologies include techniques thatattempt, from, for example, surrounding sample data and/or metadataobtained during the encoding and/or decoding of spatially neighboring,and preceding in decoding order, blocks of data. Such techniques arehenceforth called “intra prediction” techniques. Note that in at leastsome cases, intra prediction is only using reference data from thecurrent picture under reconstruction and not from reference pictures.

There can be many different forms of intra prediction. When more thanone of such techniques can be used in a given video coding technology,the technique in use can be coded in an intra prediction mode. Incertain cases, modes can have submodes and/or parameters, and those canbe coded individually or included in the mode codeword. Which codewordto use for a given mode, submode, and/or parameter combination can havean impact in the coding efficiency gain through intra prediction, and socan the entropy coding technology used to translate the codewords into abitstream.

A certain mode of intra prediction was introduced with H.264, refined inH.265, and further refined in newer coding technologies such as jointexploration model (JEM), versatile video coding (VVC), and benchmark set(BMS). A predictor block can be formed using neighboring sample valuesbelonging to already available samples. Sample values of neighboringsamples are copied into the predictor block according to a direction. Areference to the direction in use can be coded in the bitstream or maybe predicted itself

Referring to FIG. 1A, depicted in the lower right is a subset of ninepredictor directions known from H.265's 33 possible predictor directions(corresponding to the 33 angular modes of the 35 intra modes). The pointwhere the arrows converge (101) represents the sample being predicted.The arrows represent the direction from which the sample is beingpredicted. For example, arrow (102) indicates that sample (101) ispredicted from a sample or samples to the upper right, at a 45 degreeangle from the horizontal. Similarly, arrow (103) indicates that sample(101) is predicted from a sample or samples to the lower left of sample(101), in a 22.5 degree angle from the horizontal.

Still referring to FIG. 1A, on the top left there is depicted a squareblock (104) of 4 x 4 samples (indicated by a dashed, boldface line). Thesquare block (104) includes 16 samples, each labelled with an “S”, itsposition in the Y dimension (e.g., row index) and its position in the Xdimension (e.g., column index). For example, sample S21 is the secondsample in the Y dimension (from the top) and the first (from the left)sample in the X dimension. Similarly, sample S44 is the fourth sample inblock (104) in both the Y and X dimensions. As the block is 4×4 samplesin size, S44 is at the bottom right. Further shown are reference samplesthat follow a similar numbering scheme. A reference sample is labelledwith an R, its Y position (e.g., row index) and X position (columnindex) relative to block (104). In both H.264 and H.265, predictionsamples neighbor the block under reconstruction; therefore no negativevalues need to be used.

Intra picture prediction can work by copying reference sample valuesfrom the neighboring samples as appropriated by the signaled predictiondirection. For example, assume the coded video bitstream includessignaling that, for this block, indicates a prediction directionconsistent with arrow (102)—that is, samples are predicted from aprediction sample or samples to the upper right, at a 45 degree anglefrom the horizontal. In that case, samples S41, S32, S23, and S14 arepredicted from the same reference sample R05. Sample S44 is thenpredicted from reference sample R08.

In certain cases, the values of multiple reference samples may becombined, for example through interpolation, in order to calculate areference sample; especially when the directions are not evenlydivisible by 45 degrees.

The number of possible directions has increased as video codingtechnology has developed. In H.264 (year 2003), nine different directioncould be represented. That increased to 33 in H.265 (year 2013), andJEM/VVC/BMS, at the time of disclosure, can support up to 65 directions.Experiments have been conducted to identify the most likely directions,and certain techniques in the entropy coding are used to represent thoselikely directions in a small number of bits, accepting a certain penaltyfor less likely directions. Further, the directions themselves cansometimes be predicted from neighboring directions used in neighboring,already decoded, blocks.

FIG. 1B shows a schematic (105) that depicts 65 intra predictiondirections according to JEM to illustrate the increasing number ofprediction directions over time.

The mapping of intra prediction directions bits in the coded videobitstream that represent the direction can be different from videocoding technology to video coding technology; and can range, forexample, from simple direct mappings of prediction direction to intraprediction mode, to codewords, to complex adaptive schemes involvingmost probable modes, and similar techniques. In all cases, however,there can be certain directions that are statistically less likely tooccur in video content than certain other directions. As the goal ofvideo compression is the reduction of redundancy, those less likelydirections will, in a well working video coding technology, berepresented by a larger number of bits than more likely directions.

Motion compensation can be a lossy compression technique and can relateto techniques where a block of sample data from a previouslyreconstructed picture or part thereof (reference picture), after beingspatially shifted in a direction indicated by a motion vector (MVhenceforth), is used for the prediction of a newly reconstructed pictureor picture part. In some cases, the reference picture can be the same asthe picture currently under reconstruction. MVs can have two dimensionsX and Y, or three dimensions, the third being an indication of thereference picture in use (the latter, indirectly, can be a timedimension).

In some video compression techniques, an MV applicable to a certain areaof sample data can be predicted from other MVs, for example from thoserelated to another area of sample data spatially adjacent to the areaunder reconstruction, and preceding that MV in decoding order. Doing socan substantially reduce the amount of data required for coding the MV,thereby removing redundancy and increasing compression. MV predictioncan work effectively, for example, because when coding an input videosignal derived from a camera (known as natural video) there is astatistical likelihood that areas larger than the area to which a singleMV is applicable move in a similar direction and, therefore, can in somecases be predicted using a similar MV derived from MVs of a neighboringarea. That results in the MV found for a given area to be similar or thesame as the MV predicted from the surrounding MVs, and that in turn canbe represented, after entropy coding, in a smaller number of bits thanwhat would be used if coding the MV directly. In some cases, MVprediction can be an example of lossless compression of a signal(namely: the MVs) derived from the original signal (namely: the samplestream). In other cases, MV prediction itself can be lossy, for examplebecause of rounding errors when calculating a predictor from severalsurrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec.H.265, “High Efficiency Video Coding”, December 2016). Out of the manyMV prediction mechanisms that H.265 offers, described herein is atechnique henceforth referred to as “spatial merge.”

Referring to FIG. 1C, a current block (111) can include samples thathave been found by the encoder during the motion search process to bepredictable from a previous block of the same size that has beenspatially shifted. Instead of coding that MV directly, the MV can bederived from metadata associated with one or more reference pictures,for example from the most recent (in decoding order) reference picture,using the MV associated with either one of five surrounding samples,denoted A0, A1, and B0, B1, B2 (112 through 116, respectively). InH.265, the MV prediction can use predictors from the same referencepicture that the neighboring block is using.

SUMMARY

Aspects of the disclosure provide apparatuses for videoencoding/decoding. An apparatus includes processing circuitry thatdecodes prediction information of a current block in a current picturethat is a part of a coded video sequence. The prediction informationindicates a cross component filtering (CCF) process for the currentblock. The processing circuitry generates filtered reconstructionsamples of the current block by applying the CCF process on at least oneof predicted samples, residual values, or reconstruction samples of thecurrent block. The filtered reconstruction samples of the current blockare used for reconstruction of a subsequent block. The processingcircuitry reconstructs the current block and the subsequent block basedon the filtered reconstruction samples of the current block.

In one embodiment, the processing circuitry generates the filteredreconstruction samples of the current block by applying the CCF processto an output of at least one of a dequantization process and an inversetransform process of the current block.

In one embodiment, the processing circuitry reconstructs the currentblock by adding outputs of the CCF process that include offset values ofthe current block to chroma reconstruction samples of the current block.

In one embodiment, the processing circuitry reconstructs the currentblock by adding outputs of the CCF process that include offset values ofthe current block to chroma residual values of the current block.

In one embodiment, the processing circuitry reconstructs the currentblock by adding outputs of the CCF process that include offset values ofthe current block to chroma predicted samples of the current block.

In one embodiment, filter coefficients of the CCF process are includedin the prediction information.

In one embodiment, filter coefficients of the CCF process are predefinedconstants.

In one embodiment, filter coefficients of the CCF process used in thecurrent picture are determined based on filter coefficients of the CCFprocess used in another picture and offset values of the filtercoefficients of the CCF process used in the current picture.

In one embodiment, filter coefficients of the CCF process used in onecolor component are determined based on filter coefficients of the CCFprocess used in another color component.

In one embodiment, filter coefficients of the CCF process are determinedbased on sample values of the current block that are determined beforethe CCF process is applied.

In one embodiment, whether the CCF process is enabled is determinedbased on one of a partitioning scheme, a prediction mode, a block width,a block height, transform coefficients, and quantization parameters.

Aspects of the disclosure provide methods for video encoding/decoding.In the method, prediction information of a current block in a currentpicture that is a part of a coded video sequence is decoded. Theprediction information indicates a CCF process for the current block.Filtered reconstruction samples of the current block are generated byapplying the CCF process on at least one of predicted samples, residualvalues, or reconstruction samples of the current block. The filteredreconstruction samples of the current block are used for reconstructionof a subsequent block. The current block and the subsequent block arereconstructed based on the filtered reconstruction samples of thecurrent block.

Aspects of the disclosure also provide non-transitory computer-readablemediums storing instructions which when executed by at least oneprocessor cause the at least one processor to perform any one or acombination of the methods for video decoding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1A is a schematic illustration of an exemplary subset of intraprediction modes;

FIG. 1B is an illustration of exemplary intra prediction directions;

FIG. 1C is a schematic illustration of a current block and itssurrounding spatial merge candidates in one example;

FIG. 2 is a schematic illustration of a simplified block diagram of acommunication system in accordance with an embodiment;

FIG. 3 is a schematic illustration of a simplified block diagram of acommunication system in accordance with an embodiment;

FIG. 4 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment;

FIG. 5 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment;

FIG. 6 shows a block diagram of an encoder in accordance with anotherembodiment;

FIG. 7 shows a block diagram of a decoder in accordance with anotherembodiment;

FIGS. 8A-8B show exemplary diamond filter shapes of an adaptive loopfilter (ALF) in accordance with embodiments;

FIGS. 9A-9D show exemplary subsampled positions used for gradientcalculations of vertical, horizontal, and two diagonal directions,respectively, in accordance with embodiments;

FIG. 10 shows an exemplary modified block classification that is appliedto a luma component in accordance with an embodiment;

FIG. 11 shows exemplary modified ALFs for the luma component at virtualboundaries in accordance with some embodiments;

FIG. 12 shows an exemplary largest coding unit (LCU) aligned picturequadtree splitting in accordance with an embodiment;

FIG. 13 shows exemplary quadtree split flags encoded in z-order inaccordance with an embodiment;

FIG. 14A illustrates an exemplary placement of a cross-componentadaptive loop filter (CC-ALF) in accordance with an embodiment;

FIG. 14B shows an exemplary linear diamond shaped filter that is appliedto the luma channel for each chroma component during the CC-ALFoperation in accordance with an embodiment;

FIG. 15 shows an exemplary direction search for an 8×8 block inaccordance with an embodiment;

FIG. 16 shows an exemplary subspace projection in accordance with anembodiment;

FIG. 17 shows an exemplary CCF that is applied after an inversetransform of a block;

FIG. 18 shows an exemplary flowchart in accordance with an embodiment ofthe disclosure; and

FIG. 19 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

I. Video Decoder and Encoder Systems

FIG. 2 illustrates a simplified block diagram of a communication system(200) according to an embodiment of the present disclosure. Thecommunication system (200) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (250). Forexample, the communication system (200) includes a first pair ofterminal devices (210) and (220) interconnected via the network (250).In the FIG. 2 example, the first pair of terminal devices (210) and(220) performs unidirectional transmission of data. For example, theterminal device (210) may code video data (e.g., a stream of videopictures that are captured by the terminal device (210)) fortransmission to the other terminal device (220) via the network (250).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (220) may receive the codedvideo data from the network (250), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (200) includes a secondpair of terminal devices (230) and (240) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (230) and (240)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (230) and (240) via the network (250). Eachterminal device of the terminal devices (230) and (240) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (230) and (240), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 2 example, the terminal devices (210), (220), (230) and(240) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (250) represents any number ofnetworks that convey coded video data among the terminal devices (210),(220), (230) and (240), including for example wireline (wired) and/orwireless communication networks. The communication network (250) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(250) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 3 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick, and the like.

A streaming system may include a capture subsystem (313) that caninclude a video source (301), for example a digital camera, creating forexample a stream of video pictures (302) that are uncompressed. In anexample, the stream of video pictures (302) includes samples that aretaken by the digital camera. The stream of video pictures (302),depicted as a bold line to emphasize a high data volume when compared toencoded video data (304) (or coded video bitstreams), can be processedby an electronic device (320) that includes a video encoder (303)coupled to the video source (301). The video encoder (303) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (304) (or encoded video bitstream (304)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (302), can be stored on a streamingserver (305) for future use. One or more streaming client subsystems,such as client subsystems (306) and (308) in FIG. 3 can access thestreaming server (305) to retrieve copies (307) and (309) of the encodedvideo data 304). A client subsystem (306) can include a video decoder(310), for example, in an electronic device (330). The video decoder(310) decodes the incoming copy (307) of the encoded video data andcreates an outgoing stream of video pictures (311) that can be renderedon a display (312) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (304),(307), and (309) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Coding(VVC). The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (320) and (330) can includeother components (not shown). For example, the electronic device (320)can include a video decoder (not shown) and the electronic device (330)can include a video encoder (not shown) as well.

FIG. 4 shows a block diagram of a video decoder (410) according to anembodiment of the present disclosure. The video decoder (410) can beincluded in an electronic device (430). The electronic device (430) caninclude a receiver (431) (e.g., receiving circuitry). The video decoder(410) can be used in the place of the video decoder (310) in the FIG. 3example.

The receiver (431) may receive one or more coded video sequences to bedecoded by the video decoder (410); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (401), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver 431) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (431) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (415) may be coupled inbetween the receiver (431) and an entropy decoder/parser (420) (“parser(420)” henceforth). In certain applications, the buffer memory (415) ispart of the video decoder (410). In others, it can be outside of thevideo decoder (410) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (410), forexample to combat network jitter, and in addition another buffer memory(415) inside the video decoder (410), for example to handle playouttiming. When the receiver (431) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (415) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory 415) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (410).

The video decoder (410) may include the parser (420) to reconstructsymbols (421) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (410),and potentially information to control a rendering device such as arender device (412) (e.g., a display screen) that is not an integralpart of the electronic device (430) but can be coupled to the electronicdevice (430), as was shown in FIG. 4 . The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (420) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (420) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (420) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, MVs, and so forth.

The parser (420) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (415), so as tocreate symbols (421).

Reconstruction of the symbols (421) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (420). The flow of such subgroup control information between theparser (420) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (410)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (451). Thescaler/inverse transform unit (451) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (421) from the parser (420). The scaler/inversetransform unit (451) can output blocks comprising sample values that canbe input into aggregator (455).

In some cases, the output samples of the scaler/inverse transform (451)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (452). In some cases, the intra pictureprediction unit (452) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (458). The currentpicture buffer (458) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(455), in some cases, adds, on a per sample basis, the predictioninformation that the intra prediction unit (452) has generated to theoutput sample information as provided by the scaler/inverse transformunit (451).

In other cases, the output samples of the scaler/inverse transform unit(451) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (453) canaccess reference picture memory (457) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (421) pertaining to the block, these samples can beadded by the aggregator (455) to the output of the scaler/inversetransform unit (451) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (457) from where themotion compensation prediction unit (453) fetches prediction samples canbe controlled by MVs, available to the motion compensation predictionunit (453) in the form of symbols (421) that can have, for example X, Y,and reference picture components. Motion compensation also can includeinterpolation of sample values as fetched from the reference picturememory (457) when sub-sample exact MVs are in use, MV predictionmechanisms, and so forth.

The output samples of the aggregator (455) can be subject to variousloop filtering techniques in the loop filter unit (456). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (456) as symbols 421) from the parser (420), but can also beresponsive to meta-information obtained during the decoding of previous(in decoding order) parts of the coded picture or coded video sequence,as well as responsive to previously reconstructed and loop-filteredsample values.

The output of the loop filter unit (456) can be a sample stream that canbe output to the render device (412) as well as stored in the referencepicture memory (457) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (420)), the current picture buffer (458) can becomea part of the reference picture memory (457), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (410) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as documented in thevideo compression technology or standard. Specifically, a profile canselect certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (431) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (410) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 5 shows a block diagram of a video encoder (503) according to anembodiment of the present disclosure. The video encoder(503) is includedin an electronic device(520). The electronic device(520) includes atransmitter(540) (e.g., transmitting circuitry). The video encoder(503)can be used in the place of the video encoder (303) in the FIG. 3example.

The video encoder(503) may receive video samples from a videosource(501) (that is not part of the electronic device(520) in the FIG.5 example) that may capture video image(s) to be coded by the videoencoder(503). In another example, the video source(501) is a part of theelectronic device(520).

The video source(501) may provide the source video sequence to be codedby the video encoder(503) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, ...), any colorspace (for example, BT.601 Y CrCB, RGB, . . . ), andany suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (501) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (501) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (503) may code andcompress the pictures of the source video sequence into a coded videosequence (543) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller(550). In some embodiments, the controller (550)controls other functional units as described below and is functionallycoupled to the other functional units. The coupling is not depicted forclarity. Parameters set by the controller (550) can include rate controlrelated parameters (picture skip, quantizer, lambda value ofrate-distortion optimization techniques, . . . ), picture size, group ofpictures (GOP) layout, maximum MV allowed reference area, and so forth.The controller (550) can be configured to have other suitable functionsthat pertain to the video encoder (503) optimized for a certain systemdesign.

In some embodiments, the video encoder (503) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (530) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (533)embedded in the video encoder(503). The decoder(533) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (534). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (534) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder(533) can be the same as of a“remote” decoder, such as the video decoder (410), which has alreadybeen described in detail above in conjunction with FIG. 4 . Brieflyreferring also to FIG. 4 , however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (545) and the parser (420) can be lossless, the entropy decodingparts of the video decoder (410), including the buffer memory (415) andthe parser (420) may not be fully implemented in the local decoder(533).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (530) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously-coded picture fromthe video sequence that were designated as “reference pictures”. In thismanner, the coding engine (532) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (533) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (530). Operations of the coding engine (532) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 5 ), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (533) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (534). In this manner, the video encoder(503) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (535) may perform prediction searches for the codingengine (532). That is, for a new picture to be coded, the predictor(535) may search the reference picture memory (534) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture MVs, block shapes, and so on, that may serve as an appropriateprediction reference for the new pictures. The predictor (535) mayoperate on a sample block-by-pixel block basis to find appropriateprediction references. In some cases, as determined by search resultsobtained by the predictor (535), an input picture may have predictionreferences drawn from multiple reference pictures stored in thereference picture memory (534).

The controller (550) may manage coding operations of the source coder(530), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (545). The entropy coder (545)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies such as Huffman coding, variable length coding, arithmeticcoding, and so forth.

The transmitter (540) may buffer the coded video sequence(s) as createdby the entropy coder (545) to prepare for transmission via acommunication channel (560), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(540) may merge coded video data from the video coder (503) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (550) may manage operation of the video encoder (503).During coding, the controller(550) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most one MVand reference index to predict the sample values of each block.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two MVs and reference indices to predict the sample values of eachblock. Similarly, multiple-predictive pictures can use more than tworeference pictures and associated metadata for the reconstruction of asingle block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (503) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (503) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (540) may transmit additional datawith the encoded video. The source coder (530) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, SEI messages, VUI parameter setfragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) makes use of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a MV. The MV points to thereference block in the reference picture, and can have a third dimensionidentifying the reference picture, in case multiple reference picturesare in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that are both prior in decoding order to the currentpicture in the video (but may be in the past and future, respectively,in display order) are used. A block in the current picture can be codedby a first MV that points to a first reference block in the firstreference picture, and a second MV that points to a second referenceblock in the second reference picture. The block can be predicted by acombination of the first reference block and the second reference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quad-tree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels, and the like.

FIG. 6 shows a diagram of a video encoder 603) according to anotherembodiment of the disclosure. The video encoder 603) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder 603) is used in theplace of the video encoder 303) in the FIG. 3 example.

In an HEVC example, the video encoder 603) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (603) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (603) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(603) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the MV is derived from one or more MVpredictors without the benefit of a coded MV component outside thepredictors. In certain other video coding technologies, a MV componentapplicable to the subject block may be present. In an example, the videoencoder (603) includes other components, such as a mode decision module(not shown) to determine the mode of the processing blocks.

In the FIG. 6 example, the video encoder (603) includes the interencoder (630), an intra encoder (622), a residue calculator (623), aswitch (626), a residue encoder (624), a general controller (621), andan entropy encoder (625) coupled together as shown in FIG. 6 .

The inter encoder (630) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, MVs, merge mode information), and calculate inter predictionresults (e.g., prediction block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (622) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform, and in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (622) also calculates intra prediction results (e.g., predictionblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (621) is configured to determine general controldata and control other components of the video encoder (603) based onthe general control data. In an example, the general controller (621)determines the mode of the block, and provides a control signal to theswitch (626) based on the mode. For example, when the mode is the intramode, the general controller (621) controls the switch (626) to selectthe intra mode result for use by the residue calculator (623), andcontrols the entropy encoder (625) to select the intra predictioninformation and include the intra prediction information in thebitstream; and when the mode is the inter mode, the general controller(621) controls the switch (626) to select the inter prediction resultfor use by the residue calculator (623), and controls the entropyencoder (625) to select the inter prediction information and include theinter prediction information in the bitstream.

The residue calculator (623) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (622) or the inter encoder (630). Theresidue encoder (624) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (624) is configured to convert the residuedata from a spatial domain to a frequency domain, and generate thetransform coefficients. The transform coefficients are then subject toquantization processing to obtain quantized transform coefficients. Invarious embodiments, the video encoder (603) also includes a residuedecoder (628). The residue decoder (628) is configured to performinverse-transform, and generate the decoded residue data. The decodedresidue data can be suitably used by the intra encoder (622) and theinter encoder (630). For example, the inter encoder (630) can generatedecoded blocks based on the decoded residue data and inter predictioninformation, and the intra encoder (622) can generate decoded blocksbased on the decoded residue data and the intra prediction information.The decoded blocks are suitably processed to generate decoded picturesand the decoded pictures can be buffered in a memory circuit (not shown)and used as reference pictures in some examples.

The entropy encoder (625) is configured to format the bitstream toinclude the encoded block. The entropy encoder (625) is configured toinclude various information according to a suitable standard such asHEVC. In an example, the entropy encoder (625) is configured to includethe general control data, the selected prediction information (e.g.,intra prediction information or inter prediction information), theresidue information, and other suitable information in the bitstream.Note that, according to the disclosed subject matter, when coding ablock in the merge submode of either inter mode or bi-prediction mode,there is no residue information.

FIG. 7 shows a diagram of a video decoder (710) according to anotherembodiment of the disclosure. The video decoder (710) is configured toreceive coded pictures that are part of a coded video sequence, anddecode the coded pictures to generate reconstructed pictures. In anexample, the video decoder (710) is used in the place of the videodecoder (310) in the FIG. 3 example.

In the FIG. 7 example, the video decoder (710) includes an entropydecoder (771), an inter decoder (780), a residue decoder (773), areconstruction module (774), and an intra decoder (772) coupled togetheras shown in FIG. 7 .

The entropy decoder (771) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example, intramode, inter mode, bi-predicted mode, the latter two in merge submode oranother submode), prediction information (such as, for example, intraprediction information of inter prediction information) that canidentify certain sample or metadata that is used for prediction by theintra decoder 772) or the inter decoder 780), respectively, residualinformation in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isinter or bi-predicted mode, the inter prediction information is providedto the inter decoder 780); and when the prediction type is the intraprediction type, the intra prediction information is provided to theintra decoder 772). The residual information can be subject to inversequantization and is provided to the residue decoder 773).

The inter decoder 780) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder 772) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder 773) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder 773) mayalso require certain control information (to include the QuantizerParameter (QP)), and that information may be provided by the entropydecoder 771) (data path not depicted as this may be low volume controlinformation only).

The reconstruction module 774) is configured to combine, in the spatialdomain, the residual as output by the residue decoder 773) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (303), (503), and (603), and thevideo decoders (310), (410), and (710) can be implemented using anysuitable technique. In an embodiment, the video encoders (303), (503),and (603), and the video decoders (310), (410), and (710) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (303), (503), and (603), and the videodecoders (310), (410), and (710) can be implemented using one or moreprocessors that execute software instructions.

II. Adaptive Loop Filter

In some related examples such as VVC Draft 7, an adaptive loop filter(ALF) with block-based filter adaption can be applied. For a lumacomponent, one among 25 filters can be selected for each 4×4 block,based on a directionality and an activity of the respective 4×4 block.

FIGS. 8A-8B show exemplary diamond filter shapes of the ALF according tosome embodiments of this disclosure. FIG. 8A shows a 5×5 diamond shapethat can be applied to chroma components and FIG. 8B shows a 7×7 diamondshape that can be applied to a luma component.

In some related examples, for a luma component, each 4×4 block can becategorized into one out of 25 classes. A classification index C of a4×4 block can be derived based on a directionality D and a quantizedvalue of an activity Â of the 4×4 block as follows:C=5D+Â  Eq.(1)

To calculate D and Â, gradients of a horizontal, a vertical, and twodiagonal directions of the 4×4 block can first be calculated using 1-DLaplacian as follows:g _(v)=Σ_(k=i−2) ^(i+3)Σ_(l=j−2) ^(j+3) V _(k, l) , V _(k, l)=|2R(k,l)−R(k, l−1)−R(k, l+1)|  Eq. (2)g _(h)=Σ_(k=i−2) ^(i+3)Σ_(l=j−2) ⁺³ H _(K,L), h_(K,L)=|2R(k−1, l)|  (Eq.(3)g _(d1)=Σ_(k=i−2) ^(i+3)Σ_(l=j−3) ^(j+3) D1_(k,l) , D1_(k,l)=|2R(k,l)−R(k−1, l−1)−R(k+1, l+1) |  Eq.(4)g _(d2)=Σ_(k=i−2) ^(i+3)Σ_(j=j−2) ^(i+3)Σ_(j=j−2) ^(j+3) D2_(k,l) ,D2_(k,l)=|2R(k, l)−R(k−1, l+1)−R(k+1, l−1|  Eq. (5)where indices i and j refer to coordinates of upper left samples withinthe 4×4 block and R(i, j) indicates a reconstructed sample at coordinate(i, j).

To reduce the complexity of the block classification, a subsampled 1-DLaplacian calculation can be applied. FIGS. 9A-9D show exemplarysubsampled positions used for the gradient calculations of the vertical,horizontal, and two diagonal directions, respectively.

Then, the maximum and minimum values of the gradients of the horizontaland vertical directions can be set as:g _(g,v) ^(max)=max(g _(h) , g _(v))   Eq. (6)g _(h,v) ^(min)=min(g _(h) , g _(v))   Eq. (7)

The maximum and minimum values of the gradients of the two diagonaldirections can be set as:g _(d1,d2) ^(max)=max(g _(d1) , g _(d2))   Eq. (8)g _(d1,d2) ^(min)=min(g _(d1) , g _(d2))   Eq. (9)

To derive the value of the directionality D, these values can becompared against each other and with two thresholds t₁ and t₂.

Step 1: if both g_(g,v) ^(max)≤t₁·g_(h,v) ^(min) and g_(d1,d2)^(max)≤t₁·g_(d1,d2) ^(min) are true, D is set to 0.

Step 2: if g_(h,v) ^(max)/g_(h,v) ^(min)>g_(d1,d2) ^(max)/g_(d1,d2)^(min), continue from Step 3; otherwise, continue from Step 4.

Step 3: if g_(h,v) ^(max)>t₂·g_(h,v) ^(min), D is set to 2; otherwise, Dis set to 1.

Step 4: if g_(d1,d2) ^(max)>t₂·g_(d1,d2) ^(min), D is set to 4;otherwise, D is set to 3.

The activity value A is calculated as:A=Σ_(k=i−2) ^(i+3)Σ_(l=h−2) ^(j+3)(V _(k,l) +H _(k,l))   (10)

A is further quantized to a range of 0 to 4, inclusively, and thequantized value is denoted as Â.

For chroma components in a picture, no classification method is applied,i.e., a single set of ALF coefficients can be applied for each chromacomponent.

Before filtering each 4×4 luma block, geometric transformations such asrotation, diagonal flipping, and vertical flipping are applied to thefilter coefficients f(k, l) and to the corresponding filter clippingvalues c(k,l) depending on gradient values calculated for the respectiveblock. This is equivalent to applying these transformations to samplesin the filter support region, in order to make different blocks to whichthe ALF is applied more similar by aligning their directionalities.

Three geometric transformations, including diagonal flip, vertical flip,and rotation, can be described as follows:Diagonal: f _(D)(k, l)=f(l, k), c _(D), (k, l)=c(l, k)   Eq. (11)Vertical flip: f _(v)(k, l)=f(k, K−l−1), c _(V)(k,l)=c(k, K−l−1)   Eq.(12)Rotation: f _(R) (k,l)=f(K−l−1, k), c _(R)(k, l)=c(K−l−1, k)   Eq. (13)where K is a size of the filter and 0≤k, l≤K−1 are coordinates of thetransformation coefficients, such that location (0,0) is at the upperleft corner and location (K−1, K−1) is at the lower right corner. Thetransformations are applied to the filter coefficients f (k, l) and tothe clipping values c(k,l) depending on gradient values calculated forthe corresponding block. The relationship between the transformationsand the gradients of the four directions can be summarized in Table 1.

TABLE 1 Gradient values Transformation g_(d2) < g_(d1) and g_(h) < g_(v)No transformation g_(d2) < g_(d1) and g_(v) < g_(h) Diagonal g_(d1) <g_(d2) and g_(h) < g_(v) Vertical flip g_(d1) < g_(d2) and g_(v) < g_(h)Rotation

In some related examples such as VVC Draft 7, filter parameters of theALF are signaled in an adaptation parameter set (APS). In one APS, up to25 sets of luma filter coefficients and clipping value indexes, and upto eight sets of chroma filter coefficients and clipping value indexescan be signaled. To reduce bits overhead, filter coefficients ofdifferent classifications for the luma component can be merged. In aslice header, the indices of the APSs used for a current slice aresignaled. The signaling of the ALF is CTU-based in VVC Draft 7.

Clipping value indexes, which are decoded from the APS, allow clippingvalues to be determined using a table of the clipping values for theluma and chroma components. These clipping values are dependent on aninternal bit depth. For example, the table of the clipping values can beobtained by the following formula:AlfClip={round(2^(B−α*n)) for n ∈ [0 . . . N−1]}  (14)with B being equal to the internal bit depth, α being a pre-definedconstant value that is equal to 2.35, and N being equal to 4 which isthe number of allowed clipping values in VVC Draft 7. Table 2 shows anexample of the output of equation (14).

TABLE 2 clipIdx bitDepth 0 1 2 3 8 255 50 10 2 9 511 100 20 4 10 1023201 39 8 11 2047 402 79 15 12 4095 803 158 31 13 8191 1607 315 62 1416383 3214 630 124 15 32767 6427 1261 247 16 65535 12855 2521 495

In a slice header, up to 7 APS indices can be signaled to specify lumafilter sets that are used for a current slice. The filtering process canbe further controlled at a CTB level. A flag can be signaled to indicatewhether the ALF is applied to a luma CTB. The luma CTB can choose afilter set among 16 fixed filter sets and the filter sets from APSs. Afilter set index is signaled for the luma CTB to indicate which filterset is applied. The 16 fixed filter sets can be pre-defined andhard-coded in both the encoder and the decoder.

For a chroma component, an APS index can be signaled in a slice headerto indicate chroma filter sets used for a current slice. At a CTB level,a filter index can be signaled for each chroma CTB if there is more thanone chroma filter set in the APS.

The filter coefficients can be quantized with a norm being equal to 128.In order to restrict the multiplication complexity, a bitstreamconformance can be applied so that a coefficient value of a non-centralposition can be in a range of −27 to 27−1, inclusive. A central positioncoefficient is not signaled in the bitstream and is considered to beequal to 128.

In some related examples such as VVC Draft 7, syntaxes and semantics ofclipping indices and corresponding values can be defined as follows.

alf_luma_chip_idx[sfIdx][j] specifies a clipping index of a clippingvalue to be used before multiplying by the j-th coefficient of asignaled luma filter indicated by sfIdx. It is a requirement of abitstream conformance that values of alf_luma_clip_idx[sfIdx][j] withsfldx=0 . . . alf_luma_num_filters_signalled_minus1 and j=0.11 shall bein a range of 0 to 3, inclusive.

The luma filter clipping values AlfClipL[adaptation_parameter_set_id][filtIdx] with elements AlfClipL[adaptation_parameter_set_id][filtIdx][j], with filtIdx=0 . . . NumAlfFilters−1 and j=0 . . . 11 are derivedin Table 2 depending on bitDepth set equal to BitDepthY and clipldx setequal to alf_luma_clip_idx[alf_luma_coeff_delta_idx[filtIdx]][j].

alf_chroma_clip_idx[ altIdx][j] specifies a clipping index of a clippingvalue to be used before multiplying by the j-th coefficient of analternative chroma filter with index altldx. It is a requirement of abitstream conformance that values of alf_chroma_clip_idx[altldx ][j]with altldx=0 . . . alf_chroma_num_alt_filters_minus 1, j=0.5 shall bein a range of 0 to 3, inclusive.

The chroma filter clipping values

-   AlfClipC[adaptation_parameter_set_id ][altldx] with elements-   AlfClipC[adaptation_parameter_set_id][altIdx][j], with-   altldx=0 . . .alf_chroma_num_alt_filters_minus1, j=0 . . . 5 are    derived in Table 2 depending on bitDepth set equal to BitDepthC and    clipldx set equal to alf_chroma_clip_idx[altIdx][j].

At decoder side, when the ALF is enabled for a CTB, each sample R(i, j)within the CU is filtered, resulting in a corresponding sample valueR′(i, j) as shown below,

${R^{\prime}\left( {i,j} \right)} = {{R\left( {i,j} \right)} + \left( {\left( {{\sum\limits_{k \neq 0}{\sum\limits_{l \neq 0}{{f\left( {k,l} \right)} \times {K\left( {{{R\left( {{i + k},{j + l}} \right)} - {R\left( {i,j} \right)}},{c\left( {k,l} \right)}} \right)}}}} + 64} \right)\mspace{14mu}\text{>>}\mspace{14mu} 7} \right)}$where f(k, l) denotes decoded filter coefficients, K(x, y) is a clippingfunction, and c(k, l) denotes decoded clipping parameters. Variables kand 1 vary between

${- \frac{L}{2}}\mspace{14mu}{and}\mspace{14mu}\frac{L}{2}$where L denotes a filter length. The clipping function K(x, y)=min(y,max(−y, x)) which corresponds to a function Clip3 (−y, y, x). Byincorporating this clipping function, this loop filtering method becomesa non-linear process, as known as Non-Linear ALF. The selected clippingvalues are coded in the “alf data” syntax element by using a Golombencoding scheme corresponding to the index of the clipping value inTable 2. This encoding scheme is the same as the encoding scheme for thefilter index.

FIG. 10 shows an exemplary modified block classification that is appliedto a luma component according to an embodiment of the disclosure. Themodified block classification and filtering, which are employed for thesamples near horizontal CTU boundaries, can reduce the line bufferrequirement of the ALF. As shown in FIG. 10 , a virtual boundary isdefined as a line by shifting the horizontal CTU boundary with “N”samples, where N is equal to 4 for the luma component and 2 for thechroma components, respectively.

For the 1D Laplacian gradient calculation of a 4×4 block above thevirtual boundary, only samples above the virtual boundary are used.Similarly, for the 1D Laplacian gradient calculation of a 4×4 blockbelow the virtual boundary, only samples below the virtual boundary areused. The quantization of the activity value A is accordingly scaled bytaking into account the reduced number of samples used in the 1DLaplacian gradient calculation.

FIG. 11 shows exemplary modified ALFs for the luma component at virtualboundaries in accordance with some embodiments. For filteringprocessing, a symmetric padding operation at the virtual boundaries canbe used for both luma and chroma components. As shown in FIG. 11 , whena sample being filtered is located below a virtual boundary, neighboringsamples that are located above the virtual boundary are padded.Meanwhile, corresponding samples at the other side are also padded,symmetrically.

In order to enhance coding efficiency, the coding unit synchronouspicture quadtree-based ALP is used in some related examples. A lumapicture can be split into several multi-level quadtree partitions, andeach partition boundary is aligned to boundaries of largest coding units(LCUs). Each partition has its own filtering process and thus can bereferred to as a filter unit (FU).

The 2-pass encoding flow is described as follows. At the first pass, aquadtree split pattern and a best filter of each FU are decided.Filtering distortions are estimated by fast filtering distortionestimation (FFDE) during the decision process. According to the decidedquadtree split patterns and the selected filters of all FUs, thereconstructed picture is filtered. At the second pass, the CUsynchronous ALF on/off control is performed. According to the ALF on/offresults, the filtered picture from the first pass is partially recoveredby the reconstructed picture.

FIG. 12 shows an exemplary LCU aligned picture quadtree splitting inaccordance with an embodiment. A top-down splitting strategy is adoptedto divide a picture into multi-level quadtree partitions by using arate-distortion criterion. Each partition is referred to as a filterunit. The splitting process aligns the quadtree partitions with the LCUboundaries. An encoding order of the FUs follows a z-scan order. Forexample, as shown in FIG. 12 , the picture is split into 10 FUs, and theencoding order is FU0, FU1, FU2, FU3, FU4, FUS, FU6, FU7, FU8, and FU9.

FIG. 13 shows an exemplary quadtree split pattern in correspondence withFIG. 12 . To indicate the picture quadtree split pattern, split flagscan be encoded and transmitted in z-order.

The filter of each FUcan beselected from two filter sets based on therate-distortion criterion. The first set has 1/2-symmetric square-shapedand rhombus-shaped filters newly derived for the current FU. The secondset comes from time-delayed filter buffers which store the filterspreviously derived for FUs of prior pictures. The filter with theminimum rate-distortion cost of these two sets can be chosen for thecurrent FU. Similarly, if the current FU is not the smallest FU and canbe further split into 4 children FUs, the rate-distortion costs of the 4children FUs are calculated. By comparing the rate-distortion cost ofthe split and non-split cases recursively, the picture quadtree splitpattern can be decided.

In some related examples, the maximum quadtree split level is 2, whichmeans the maximum number of FUs is 16. During the quadtree splitdecision, the correlation values for deriving Wiener coefficients of the16 FUs at a bottom quadtree level (smallest FUs) can be reused. The restFUs can derive their Wiener filters from the correlations of the 16FUsat the bottom quadtree level. Therefore, there is only one frame bufferaccess for deriving the filter coefficients of all FUs.

After the quadtree split pattern is decided, to further reduce thefiltering distortion, the CU synchronous ALF on/off control can beperformed. By comparing the filtering distortion and non-filteringdistortion, the leaf CU can explicitly switch the ALF on/off in itslocal region. The coding efficiency may be further improved byredesigning the filter coefficients according to the ALF on/off results.However, the redesigning process needs additional frame buffer accesses.In some related examples, there is no redesign process after the CUsynchronous ALF on/off decision in order to minimize the number of framebuffer accesses.

III. Cross Component Adaptive Loop Filter

In some related examples, a cross-component adaptive loop filter(CC-ALF) is employed. The CC-ALF makes use of luma sample values torefine each chroma component.

FIG. 14A illustrates an exemplary placement of the CC-ALF according toan embodiment of the disclosure. FIG. 14B shows an exemplary lineardiamond shaped filter that is applied to the luma channel for eachchroma component during the CC-ALF operation. The filter coefficientscan be transmitted in the APS, for example scaled by a factor of 2¹⁰ androunded for fixed point representation. The application of the filtersis controlled on a variable block size and signaled by a context-codedflag received for each block of samples. The block size along with aCC-ALF enabling flag is received at the slice-level for each chromacomponent. In an example, the following block sizes (in chroma samples)are supported: 16×16, 32×32, and 64×64.

Table 3 shows syntax elements related to the CC-ALF.

TABLE 3 if ( slice_cross_component_alf_cb_enabled_flag )alf_ctb_cross_component_cb_idc[ xCtb >> CtbLog2SizeY ][ yCtb >>CtbLog2SizeY ] ae(v) if( slice_cross_component_alf_cb_enabled_flag = = 0∥ alf_ctb_cross_component_cb_idc[ x Ctb >> CtbLog2SizeY ][ yCtb >>CtbLog2SizeY ] == 0 ) if( slice_alf_chroma_idc = = 1 | |slice_alf_chroma_idc = = 3 ) { alf_ctb_flag[ 1 ][ xCtb >> CtbLog2SizeY][ yCtb >> CtbLog2SizeY ] ae(v) if( alf_ctb_flag[ 1 ][ xCtb >>CtbLog2SizeY ][ yCtb >> CtbLog2SizeY ] &&aps_alf_chroma_num_alt_filters_minus1 > 0 ) alf_ctb_filter_alt_idx[ 0 ][xCtb >> CtbLog2SizeY ][ yCtb >> CtbLog2SizeY ] ae(v) } if (slice_cross_component_alf_cr_enabled_flag )alf_ctb_cross_component_cr_idc[ xCtb >> CtbLog2SizeY ][ yCtb >>CtbLog2SizeY ] ae(v) if( slice_cross_component_alf_cr_enabled_flag = = 0∥ alf_ctb_cross_component_cr_idc[ xC tb >> CtbLog2SizeY ][ yCtb >>CtbLog2SizeY ] == 0 ) if( slice_alf_chroma_idc = = 2 | |slice_alf_chroma_idc = = 3 ) { alf_ctb_flag[ 2 ][ xCtb >> CtbLog2SizeY][ yCtb >> CtbLog2SizeY ] ae(v) if( alf_ctb_flag[ 2 ][ xCtb >>CtbLog2SizeY ][ yCtb >> CtbLog2SizeY ] &&aps_alf_chroma_num_alt_filters_minus1 > 0 ) alf_ctb_filter_alt_idx[ 1 ][xCtb >> CtbLog2SizeY ][ yCtb >> CtbLog2SizeY ] ae(v) }

In Table 3, the syntax element alf_ctb_cross_component_cb_idc[xCtb>>CtbLog 2SizeY ][yCtb>>Ctb Log 2SizeY] equal to 0 indicates that the crosscomponent Cb filter is not applied to a block of samples in Cb colorcomponent at a luma location (xCtb, yCtb). The syntax element

-   alf_tb_css_component_ct_idc[xCtb >>Ctb Log 2SizeY ][yCtb >>Ctb    Log2SizeY] not equal to 0 indicates that the-   alf_ctb_cross_component_cr idvc[xCtb >>Ctb Log 2SizeY ][yCtb >>Ctb    Log 2SizeY ]-th cross component Cb filter is applied to the block of    samples in Cb color component at the luma location (xCtb, yCtb).

IV. Constrained Directional Enhancement Filter

One goal of an in-loop constrained directional enhancement filter (CDEF)is to filter out coding artifacts while retaining details of an image.In some related examples such as HEVC, a sample adaptive offset (SAO)algorithm can achieve a similar goal by defining signal offsets fordifferent classes of pixels. Unlike SAO, the CDEF is a non-linearspatial filter. The design of the filter has been constrained to beeasily vectorizable, such as implementable with single instructionmultiple data (SIMD) operations, which is not the case for othernon-linear filters such as a median filter and a bilateral filter.

The CDEF design originates from the following observations. The amountof ringing artifacts in a coded image tends to be roughly proportionalto a quantization step size. The amount of detail is a property of theinput image, but the smallest detail retained in the quantized imagetends to also be proportional to the quantization step size. For a givenquantization step size, an amplitude of the ringing is generally lessthan amplitudes of the details.

The CDEF works by identifying a direction of each block and thenadaptively filtering along the identified direction and to a lesserdegree along directions rotated 45 degrees from the identifieddirection. Filter strengths are signaled explicitly, which allow a highdegree of control over the blurring. An efficient encoder search isdesigned for the filter strengths. The CDEF is based on two previouslyproposed in-loop filters and the combined filter is adopted for theemerging AOMedia Video 1 (AV1) codec.

FIG. 15 shows an exemplary direction search for an 8×8 block accordingto an embodiment of the disclosure. The direction search operates on thereconstructed pixels, just after a deblocking filter. Since those pixelsare available to the decoder, the directions require no signaling. Thesearch operates on 8×8 blocks, which are small enough to adequatelyhandle non-straight edges, while being large enough to reliably estimatedirections when being applied to a quantized image. Having a constantdirection over an 8×8 region also makes vectorization of the filtereasier. For each block the direction that best matches the pattern inthe respective block is determined by minimizing a sum of squareddifferences (SSD) between the quantized block and the closest perfectlydirectional block. A perfectly directional block is a block where all ofthe pixels along a line in one direction have the same value.

One reason for identifying the direction is to align the filter tapsalong that direction to reduce ringing while preserving the directionaledges or patterns. However, directional filtering alone sometimes cannotsufficiently reduce ringing. It is also desired to use filter taps onpixels that do not lie along the main direction. To reduce the risk ofthe blurring, these extra taps are treated more conservatively. For thisreason, the CDEF defines primary taps and secondary taps. The complete2-D CDEF filter is expressed as follows:y(i,j)=x(i,j)+round(Σ_(m,n) w _(d,m,n) ^((p)) f(x(m,n)−x(i,j),S ^((p)),D)+Σ_(m,n) w _(d,m,n) ^((s)) f(x)m,n)−x(i,j),S ^((s)) ,D))   Eq. (16)where D is a damping parameter, S^((p)) and S^((s)) are the strengths ofthe primary and secondary taps, respectively, and round(·)rounds tiesaway from zero, w_(d,m,n) ^((p)) and w_(d,m,n) ^((s)) are filter weightsand f(d, S, D) is a constraint function operating on a differencebetween the filtered pixel and each of the neighboring pixels. For asmall difference, f(d, S, D)=d, making the filter behave like a linearfilter. When the difference is large, f(d, S, D)=0, which effectivelyignores the filter tap.

V. Loop Restortation In AV1

A set of in-loop restoration schemes can be used in video coding postdeblocking, to generally de-noise and enhance the quality of edges,beyond a traditional deblocking operation. These schemes are switchablewithin a frame per suitably sized tile. The specific schemes describedare based on separable symmetric Wiener filters and dual self-guidedfilters with subspace projection. Because content statistics can varysubstantially within a frame, these tools are integrated within aswitchable framework where different tools can be triggered in differentregions of the frame.

For a Wiener filter, every pixel in a degraded frame can bereconstructed as a non-causal filtered version of the respective pixelwithin a w x w window around the respective pixel, where w=2r +1 is oddfor integer r. If the 2D filter taps are denoted by a w²×1 elementvector F in a column-vectorized form, a straightforward linear minimummean square error (LMMSE) optimization leads to filter parameters beinggiven by F=H⁻¹ M, where H=E [XX^(T)] is the autocovariance of x, thecolumn-vectorized version of the w² samples in the w×w window around apixel, and M=E[YX] is the cross correlation of x with the scalar sourcesample y, to be estimated. The encoder can estimate H and M fromrealizations in the deblocked frame and the source, and send theresultant filter F to the decoder. However, this would not only incur asubstantial bit rate cost in transmitting w² taps, but also anon-separable filtering making decoding prohibitively complex.Therefore, several additional constraints are imposed on the nature ofF. First, F is constrained to be separable so that the filtering can beimplemented as separable horizontal and vertical w-tap convolutions.Second, each of the horizontal and vertical filters is constrained to besymmetric. Third, a sum of both the horizontal and vertical filtercoefficients is assumed to be 1.

A local linear model of guided filtering can be expressed as follows:y=Fx+G   Eq. (17)

The local linear model is used to compute a filtered outputy from anunfiltered sample x, where F and G are determined based on statistics ofthe degraded image and a guidance image in the neighborhood of thefiltered pixel. If the guide image is the same as the degraded image,the resultant so-called self-guided filtering has an effect of edgepreserving smoothing. A specific form of the self-guided filteringdepends on two parameters: a radius r and a noise parameter e, and isenumerated as follows.

(1) Obtain mean μ and variance σ² of pixels in a (2r+1)×(2r+1) windowaround every pixel. This can be implemented efficiently with a boxfiltering based on integral imaging.

(2) Compute for every pixel: f=σ²/σ²+e); g=(1−f)μ.

(3) Compute F and G for every pixel as averages off and g values in a3×3 window around the pixel for use.

Filtering is controlled by r and e, where a higher r implies a higherspatial variance and a higher e implies a higher range variance.

FIG. 16 shows an exemplary subspace projection in accordance with anembodiment. Even though none of the cheap restorations X₁ and X₂ areclose to the source Y, appropriate multipliers {α, β} can bring themmuch closer to the source as long as they are moving somewhat in theright direction.

VI. Cross-Component Filtering On Block-level Reconstruction

In some related examples such as VVC, a cross-component filtering (CCF)process has been proposed and adopted as an additional filtering processafter SAO. That is, the CCF process is applied outside the transform,quantization, dequantization, and reconstruction loop. Therefore, theCCF process that is applied on a current block has no improvement onpredictions of subsequent coding blocks since reconstruction samples ofthe current block that are used as reference samples for the subsequentblocks are not impacted by the CCF process. In such examples, the CCFprocess is applied on a picture level. In order for the CCF process toaffect the prediction of the subsequent coding blocks, the CCF processcan be applied on a block level in embodiments of this disclosure.

This disclosure includes methods of applying the CCF process onblock-level reconstruction. The CCF process can be defined as afiltering process which uses reconstructed samples of a first colorcomponent as inputs (e.g., Y or Cb or Cr), and outputs can be applied ona second color component different from the first color component. Oneexample of the CCF process is the CC-ALF process as described in sectionIII.

According to aspects of the disclosure, the CCF process can be appliedbefore the SAO is completed. For example, the CCF process can be appliedafter the dequantization and/or the inverse transform. The filteredreconstruction samples of the current block output by the CCF processcan be used as reference samples for subsequent blocks and/or used forgenerating prediction samples of the subsequent blocks.

FIG. 17 shows an exemplary CCF that is applied after an inversetransform of a block according to an embodiment of the disclosure. Thefiltered samples can be used for intra prediction and/or interprediction of the subsequent blocks, and in-loop filtering can beapplied on top of the filtered samples that are output from the CCFprocess.

In one embodiment, the CCF can be only applied to chroma colorcomponents. The inputs of the CCF process can be reconstructed lumasamples and the outputs can be offset values that are added on top ofchroma samples of the current block.

In one embodiment, the CCF process can be applied on residuals. Theinputs of the CCF process can be luma residual values derived from theinverse transform, and the outputs can be offsets values that are addedon chroma residual values of the current block.

In one embodiment, the CCF process can be applied on predicted samplevalues. The inputs of the CCF process can be applied on predicted lumasample values and the outputs can be offset values that are added onpredicted chroma sample values of the current block.

In one embodiment, filter coefficients used in the CCF process can besignaled. For example, the filter coefficients can be signaled in thevideo parameter set (VPS), sequence parameter set (SPS), pictureparameter set (PPS), adaptation parameter set (APS), slice header, ortile header.

In one embodiment, filter coefficients used in the CCF process arepre-defined constants.

In one embodiment, filter coefficients used in the CCF process for acurrent picture are derived by filter coefficients used in another CCFprocess. For example, the filter coefficients used in the CCF processfor the current picture can be derived by filter coefficients used inthe CCF process for a different picture and offset values that aresignaled for the current picture.

In one embodiment, filter coefficients used in the CCF process for onecolor component (e.g., chroma component) are derived from filtercoefficients used in the CCF process for the other color component(e.g., luma component).

In one embodiment, filter coefficients are selected for each sampleaccording to some statistics that are derived by sample valuesdetermined before the CCF process is applied.

In some embodiments, a determination is made as to whether the CCFprocess is enabled. The CCF process is applied based on a determinationthat the CCF process is enabled.

In one embodiment, the CCF process may be enabled or disabled forcertain block sizes. In one example, the CCF process is disabled for ablock with a width and/or a height being smaller than a given thresholdvalue.

In one embodiment, the CCF process may be enabled or disabled forcertain partitioning schemes. In one example, the CCF process isdisabled when luma and chroma components have different block sizes,such as when luma and chroma components have different partitioningschemes or semi-decoupled partitioning schemes.

In one embodiment, the CCF process may be enabled or disabled forcertain prediction modes. In one example, the CCF process is disabledfor DC mode. In one example, the CCF process is disabled for Planarmode. In one example, the CCF process is disabled for one or multiple ofSMOOTH, SMOOTH_H, and SMOOTH_V modes. In one example, the CCF process isdisabled for Paeth predictor mode. In one example, the CCF process isdisabled for SKIP mode. In one example, the CCF process is enabled fordirectional mode. In one example, the CCF process is enabled forinter-intra compound mode.

In one embodiment, a filter shape of the CCF process may depend on theblock width and/or height.

In one embodiment, whether the CCF process is enabled depends ontransform coefficients of the inverse transform process. In one example,if the transform coefficients (before dequantization or afterdequantization) are all zero, the CCF process is not applied. In oneexample, if only the DC transform coefficients (before dequantization orafter dequantization) are nonzero, the CCF process is not applied. Inone example, if only the low-frequency transform coefficients (beforedequantization or after dequantization) are nonzero, the CCF process isnot applied. The low-frequency transform coefficients are transformcoefficients that are located at a coordinate (x, y) with x and/or ybeing smaller than a given threshold.

In one embodiment, whether the CCF process is enabled depends onquantization parameters of the dequantization process.

VII. Flowchart

FIG. 18 shows a flow chart outlining an exemplary process (1800)according to an embodiment of the disclosure. In various embodiments,the process (1800) is executed by processing circuitry, such as theprocessing circuitry in the terminal devices (210), (220), (230) and(240), the processing circuitry that performs functions of the videoencoder (303), the processing circuitry that performs functions of thevideo decoder (310), the processing circuitry that performs functions ofthe video decoder (410), the processing circuitry that performsfunctions of the intra prediction module (452), the processing circuitrythat performs functions of the video encoder (503), the processingcircuitry that performs functions of the predictor (535), the processingcircuitry that performs functions of the intra encoder (622), theprocessing circuitry that performs functions of the intra decoder (772),and the like. In some embodiments, the process (1800) is implemented insoftware instructions, thus when the processing circuitry executes thesoftware instructions, the processing circuitry performs the process1800).

The process 1800) may generally start at step (S1810), where the process(1800) decodes prediction information of a current block in a currentpicture that is a part of a coded video sequence. The predictioninformation indicates a CCF process for the current block. Then, theprocess (1800) proceeds to step (S1820).

At step (S1820), the process (1800) generates filtered reconstructionsamples of the current block by applying the CCF process on at least oneof predicted samples, residual values, or reconstruction samples of thecurrent block. The filtered reconstruction samples of the current blockare used for reconstruction of a subsequent block. Then, the process(1800) proceeds to step (S1830).

At step (S1830), the process (1800) reconstructs the current block andthe subsequent block based on the filtered reconstruction samples of thecurrent block. Then, the process (1800) terminates.

In one embodiment, the process (1800) generates the filteredreconstruction samples of the current block by applying the CCF processto an output of at least one of a dequantization process and an inversetransform process of the current block.

In one embodiment, the process (1800) reconstructs the current block byadding outputs of the CCF process that include offset values of thecurrent block to chroma reconstruction samples of the current block.

In one embodiment, the process (1800) reconstructs the current block byadding outputs of the CCF process that include offset values of thecurrent block to chroma residual values of the current block.

In one embodiment, the process (1800) reconstructs the current block byadding outputs of the CCF process that include offset values of thecurrent block to chroma predicted samples of the current block.

In one embodiment, filter coefficients of the CCF process are includedin the prediction information.

In one embodiment, filter coefficients of the CCF process are predefinedconstants.

In one embodiment, filter coefficients of the CCF process used in thecurrent picture are determined based on filter coefficients of the CCFprocess used in another picture and offset values of the filtercoefficients of the CCF process used in the current picture. The offsetvalues can be included in the prediction information.

In one embodiment, filter coefficients of the CCF process used in onecolor component are determined based on filter coefficients of the CCFprocess used in another color component.

In one embodiment, the filter coefficients of the CCF process aredetermined based on sample values of the current block that aredetermined before the CCF process is applied.

In one embodiment, whether the CCF process is enabled is determinedbased on one of a partitioning scheme, a prediction mode, a block width,a block height, transform coefficients, and quantization parameters.

VIII. Computer System

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 19 shows a computersystem (1900) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 19 for computer system (1900) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1900).

Computer system (1900) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1901), mouse (1902), trackpad (1903), touchscreen (1910), data-glove (not shown), joystick (1905), microphone(1906), scanner (1907), and camera (1908).

Computer system (1900) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1910), data-glove (not shown), or joystick (1905), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1909), headphones(not depicted)), visual output devices (such as screens (1910) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted). These visual output devices (such as screens(1910)) can be connected to a system bus (1948) through a graphicsadapter (1950).

Computer system (1900) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1920) with CD/DVD or the like media (1921), thumb-drive (1922),removable hard drive or solid state drive (1923), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1900) can also include a network interface (1954) toone or more communication networks (1955). The one or more communicationnetworks (1955) can for example be wireless, wireline, optical. The oneor more communication networks (1955) can further be local, wide-area,metropolitan, vehicular and industrial, real-time, delay-tolerant, andso on. Examples of the one or more communication networks (1955) includelocal area networks such as Ethernet, wireless LANs, cellular networksto include GSM, 3G, 4G, 5G, LTE and the like, TV wireline or wirelesswide area digital networks to include cable TV, satellite TV, andterrestrial broadcast TV, vehicular and industrial to include CANBus,and so forth. Certain networks commonly require external networkinterface adapters that attached to certain general purpose data portsor peripheral buses (1949) (such as, for example USB ports of thecomputer system (1900)); others are commonly integrated into the core ofthe computer system (1900) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface into a smartphone computer system). Using anyof these networks, computer system (1900) can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1940) of thecomputer system (1900).

The core (1940) can include one or more Central Processing Units (CPU)(1941), Graphics Processing Units (GPU) (1942), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1943), hardware accelerators for certain tasks (1944), graphicsadapters (1950), and so forth. These devices, along with Read-onlymemory (ROM) (1945), Random-access memory (1946), internal mass storage(1947) such as internal non-user accessible hard drives, SSDs, and thelike, may be connected through the system bus (1948). In some computersystems, the system bus (1948) can be accessible in the form of one ormore physical plugs to enable extensions by additional CPUs, GPU, andthe like. The peripheral devices can be attached either directly to thecore's system bus (1948), or through a peripheral bus (1949). In anexample, the screen (1910) can be connected to the graphics adapter(1950). Architectures for a peripheral bus include PCI, USB, and thelike.

CPUs (1941), GPUs (1942), FPGAs (1943), and accelerators (1944) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1945) or RAM (1946). Transitional data can be also be stored in RAM(1946), whereas permanent data can be stored for example, in theinternal mass storage (1947). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1941), GPU (1942), massstorage (1947), ROM (1945), RAM (1946), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (1900), and specifically the core (1940) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (1940) that are of non-transitorynature, such as core-internal mass storage (1947) or ROM (1945). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1940). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1940) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1946) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1944)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

Appendix A: Acronyms

-   ALF: Adaptive Loop Filter-   AMVP: Advanced Motion Vector Prediction-   APS: Adaptation Parameter Set-   ASIC: Application-Specific Integrated Circuit-   ATMVP: Alternative/Advanced Temporal Motion Vector Prediction-   AV1: AOMedia Video 1-   AV2: AOMedia Video 2-   MS: Benchmark Set-   BvV: Block Vector-   CANBus: Controller Area Network Bus-   CB: Coding Block-   CC-ALF: Cross-Component Adaptive Loop Filter-   CD: Compact Disc-   CDEF: Constrained Directional Enhancement Filter-   CPR: Current Picture Referencing-   CPUs: Central Processing Units-   CRT: Cathode Ray Tube-   CTBs: Coding Tree Blocks-   CTUs: Coding Tree Units-   CU: Coding Unit-   DPB: Decoder Picture Buffer-   DPS: Decoding Parameter Set-   DVD: Digital Video Disc-   FPGA: Field Programmable Gate Areas-   JCCR: Joint CbCr Residual Coding-   JVET: Joint Video Exploration Team-   GOPs: Groups of Pictures-   GPUs: Graphics Processing Units-   GSM: Global System for Mobile communications-   HDR: High Dynamic Range-   HEVC: High Efficiency Video Coding-   HRD: Hypothetical Reference Decoder-   IBC: Intra Block Copy-   IC: Integrated Circuit-   ISP: Intra Sub-Partitions-   JEM: Joint Exploration Model-   LAN: Local Area Network-   LCD: Liquid-Crystal Display-   LR: Loop Restoration Filter-   LTE: Long-Term Evolution-   MPM: Most Probable Mode-   MV: Motion Vector-   OLED: Organic Light-Emitting Diode-   PBs: Prediction Blocks-   PCI: Peripheral Component Interconnect-   PDPC: Position Dependent Prediction Combination-   PLD: Programmable Logic Device-   PPS: Picture Parameter Set-   PUs: Prediction Units-   RAM: Random Access Memory-   ROM: Read-Only Memory-   SAO: Sample Adaptive Offset-   SCC: Screen Content Coding-   SDR: Standard Dynamic Range-   SEI: Supplementary Enhancement Information-   SNR: Signal Noise Ratio-   SPS: Sequence Parameter Set-   SSD: Solid-state DrivevTUs: Transform Units-   USB: Universal Serial Bus-   VPS: Video Parameter Set-   VUI: Video Usability Information-   VVC: Versatile Video Coding-   WAIP: Wide-Angle Intra Prediction

What is claimed is:
 1. A method of image or video decoding in a decoder,comprising: decoding prediction information of a current block in acurrent picture that is a part of a coded video sequence, the predictioninformation indicating a cross component filtering (CCF) process for thecurrent block; generating filtered reconstruction samples of the currentblock by applying the CCF process on at least one of predicted samples,residual values, or reconstruction samples of the current block, thefiltered reconstruction samples of the current block being used forreconstruction of a subsequent block; and reconstructing the currentblock and the subsequent block based on the filtered reconstructionsamples of the current block, wherein filter coefficients of the CCFprocess used in one color component are derived from filter coefficientsof the CCF process used in another color component.
 2. The method ofclaim 1, wherein the generating includes generating the filteredreconstruction samples of the current block by applying the CCF processto an output of at least one of a dequantization process and an inversetransform process of the current block.
 3. The method of claim 1,wherein the reconstructing includes reconstructing the current block byadding outputs of the CCF process that include offset values of thecurrent block to chroma reconstruction samples of the current block. 4.The method of claim 1, wherein the reconstructing includesreconstructing the current block by adding outputs of the CCF processthat include offset values of the current block to chroma residualvalues of the current block.
 5. The method of claim 1, wherein thereconstructing includes reconstructing the current block by addingoutputs of the CCF process that include offset values of the currentblock to chroma predicted samples of the current block.
 6. The method ofclaim 1, wherein the filter coefficients of the CCF process used in theother color component are included in the prediction information.
 7. Themethod of claim 1, wherein the filter coefficients of the CCF processused in the other color component are predefined constants.
 8. Themethod of claim 1, wherein the filter coefficients of the CCF processused in the current picture are determined based on filter coefficientsof the CCF process used in another picture and offset values of thefilter coefficients of the CCF process used in the current picture. 9.The method of claim 1, wherein the filter coefficients of the CCFprocess are determined based on sample values of the current block thatare determined before the CCF process is applied.
 10. The method ofclaim 1, wherein whether the CCF process is enabled is determined basedon one of a partitioning scheme, a prediction mode, a block width, ablock height, transform coefficients, and quantization parameters. 11.An apparatus, comprising processing circuitry configured to: decodeprediction information of a current block in a current picture that is apart of a coded video sequence, the prediction information indicating across component filtering (CCF) process for the current block; generatefiltered reconstruction samples of the current block by applying the CCFprocess on at least one of predicted samples, residual values, orreconstruction samples of the current block, the filtered reconstructionsamples of the current block being used for reconstruction of asubsequent block; and reconstruct the current block and the subsequentblock based on the filtered reconstruction samples of the current block,wherein filter coefficients of the CCF process used in one colorcomponent are derived from filter coefficients of the CCF process usedin another color component.
 12. The apparatus of claim 11, wherein theprocessing circuitry is configured to: generate the filteredreconstruction samples of the current block by applying the CCF processto an output of at least one of a dequantization process and an inversetransform process of the current block.
 13. The apparatus of claim 11,wherein the processing circuitry is configured to: reconstruct thecurrent block by adding outputs of the CCF process that include offsetvalues of the current block to chroma reconstruction samples of thecurrent block.
 14. The apparatus of claim 11, wherein the processingcircuitry is configured to: reconstruct the current block by addingoutputs of the CCF process that include offset values of the currentblock to chroma residual values of the current block.
 15. The apparatusof claim 11, wherein the processing circuitry is configured to:reconstruct the current block by adding outputs of the CCF process thatinclude offset values of the current block to chroma predicted samplesof the current block.
 16. The apparatus of claim 11, wherein the filtercoefficients of the CCF process used in the other color component areincluded in the prediction information.
 17. The apparatus of claim 11,wherein the filter coefficients of the CCF process used in the othercolor component are predefined constants.
 18. The apparatus of claim 11,wherein the filter coefficients of the CCF process used in the currentpicture are determined based on filter coefficients of the CCF processused in another picture and offset values of the filter coefficients ofthe CCF process used in the current picture.
 19. A non-transitorycomputer-readable storage medium storing instructions which whenexecuted by at least one processor cause the at least one processor toperform: decoding prediction information of a current block in a currentpicture that is a part of a coded video sequence, the predictioninformation indicating a cross component filtering (CCF) process for thecurrent block; generating filtered reconstruction samples of the currentblock by applying the CCF process on at least one of predicted samples,residual values, or reconstruction samples of the current block, thefiltered reconstruction samples of the current block being used forreconstruction of a subsequent block; and reconstructing the currentblock and the subsequent block based on the filtered reconstructionsamples of the current block, wherein filter coefficients of the CCFprocess used in one color component are derived from filter coefficientsof the CCF process used in another color component.